The invention relates to a mail handling system which prints information on a mailpiece, and more particularly to an apparatus and method for accurately controlling printing on a mailpiece in a mail handling system which transports mailpieces of varying size and weight at a high rate of speed.
Known postage meters are utilized in lieu of postage stamps as a means for printing an indication of paid postage (indicium) on a mailpiece. Additionally, mailing machines which perform some or all of the functions of transporting, separating, sealing, and weighing mailpieces at a high throughput are also known. When a mailing machine is combined with a postage meter so that the mailpieces being processed through the mailing machine have the indicium printed thereon, a mail handling system is created.
In recent years, the technological advancement in digital printing techniques has led to the use of digital printheads in postage meters as a preferred means for printing the indicium and other information on mailpieces. Digital printers, such as ink jet or laser printers, provide advantages over previously used rotary drum printing systems in that they are relatively inexpensive, easy to replace, and readily adaptable to print various images simply through the use of new or upgraded software. However, in a high speed mail handling system which, for example, can process 240 envelopes per minute, the control of the energizing of the digital printhead in synchronization with the movement of the mailpieces is essential for producing a good quality printed image at the required location on the mailpiece.
U.S. Pat. No. 5,561,103, issued to Arsenault et al., (hereinafter referred to as the '103 patent" and which is incorporated herein by reference), is directed to a low cost stand alone postage meter which incorporates an ink jet printhead and which is not connected to a mailing machine. In the '103 patent apparatus a user inserts a mailpiece into the postage meter and the mailpiece is then clamped in place. The inkjet printhead is then over the clamped mailpiece and prints the indicium on the mailpiece which is subsequently removed by the operator. In this apparatus, an Application Specific Integrated Circuit (ASIC) is utilized to draw indicium image data from a memory device and provides this image data to the printhead on an indicium image column by column basis in a synchronous manner. Since it is assumed that the printhead is being moved at a fairly constant velocity during printing and the image data is synchronously transmitted by the ASIC, the printhead fire pulse is fixed accordingly. Thus, in the '103 patent apparatus, as each column of image data is transmitted by the ASIC it is subsequently printed by the printhead before the next column of data is transmitted by the ASIC. This is referred to as real time column by column or draw on the fly printing whereby no more than one column of the image data is ever built and stored by the ASIC at one time.
It is quite clear from the above description of the '103 apparatus that since the ASIC transmits data synchronously in accordance with its own internal clock pulses, the energizing of the printhead to print each column of image data must occur within the pulse range of the ASIC for each block of column data in order to prevent corruption of the image data. In an ideal world, if the velocity of the printhead movement was controlled without variation, the firing pulse to the printhead could be fixed. However, there is some variation in the pulsing of the stepper motor which controls the printhead movement such that in the '103 apparatus means for adjusting the fire pulse rate to the printhead to accommodate such pulse variation is provided for as set forth in U.S. patent application Ser. No. 08/707,984 filed on Aug. 23, 1996. However, because the mailpiece medium is fixed and only the printhead is being moved, the variation in the velocity of the printhead is within plus or minus 3% of the ideal printhead velocity. This tolerance is small enough to be accommodated within the pulse width of the ASIC data transmission such that the minor adjustments to the printhead fire pulse do not create any data corruption.
A key benefit of using a fixed rate column by column image generator (ASIC) in the '103 apparatus is that the need for building, editing and storing the entire indicium image is not required such the throughput capability of the postage meter is increased. Moreover, since a dedicated RAM is not required to store the entire bit map image of the complete indicium, a reduced cost benefit is also achieved.
A known alternative to the digital image generating engine described above is set forth in U.S. Pat. No. 5,608,636. However, in the apparatus of the '636 patent a bit map of the entire indicium image for each individual postage transaction is first built and stored in a RAM prior to printing the indicium image on the mailpiece. Since the indicium image includes both fixed and variable data, a great deal of editing is required to build the entire image into the RAM on a mailpiece by mailpiece basis. This entire image building process takes time which either reduces the throughput of the printing process or requires the use of costly high-speed microprocessors to help improve printing throughput.
In view of the problems discussed above in connection with the apparatus of the '536 patent, the instant inventors desired to utilize an image generating engine similar to the one used in the '103 patent in a mail handling system capable of processing mixed size and weight mailpieces at high throughput speeds. Furthermore, since the ASIC described in the '103 patent was already implemented in the Personal Post Office.TM. postage meter product being sold by the employer, Pitney Bowes Inc., of the instant inventors, the reuse of the '103 ASIC was also desirable to reduce engineering development time, provide commonality between product lines, and achieve a reduction in product cost based on reduced costs realized by the ordering of larger quantities of a common component utilized in multiple products. However, the mail handling system that the inventors were concerned with was different from the stand alone postage meter of the '103 apparatus in that a stationary printhead was used to print the indicium on mailpieces being transported by the printhead at a high processing speed. Since the mailpieces being transported through the mail handling machine may vary significantly in weight, a variation in the rate at which the mailpieces will pass by the printhead can be as much as plus or minus 15% which is greater than the variation in the printhead speed of the apparatus of the '103 patent. Moreover, the encoder used in the mail handling system for synchronizing the energizing of the printhead with mailpiece movement is a quadrature output encoder where two output channels are utilized as a cost saving measure to provide the desired encoder count capability. Since two output channels are being used in this manner each output channel has an edge to edge tolerance such that the combined output encoder pulse edge to edge tolerance is approximately plus or minus 15% Accordingly, in the mail handling system that the inventors were concerned with a variation between encoder pulses could be as much as plus or minus 30% from the desired encoder pulse rate. Since the energizing of the printhead is directly associated with each encoder pulse, it became evident to the inventors that while the ASIC provided image data to the printhead at a fixed rate the use of the image data at the printhead was asynchronous due to the above discussed variation in encoder pulse frequency. However, unlike the apparatus of '103 patent, the 30% variation between encoder pulses could not be accommodated within the data producing pulse width of the image data generating ASIC. Thus, if the printhead utilizes the ASIC produced image data at a slower rate than it is generated by the ASIC the data becomes corrupted since the single column data buffer will be receiving the next column of data before the previous column of image data has been provided to the printhead. On the other hand, if due to the encoder tolerances the printhead utilizes the ASIC produced image data at a rate faster than the ASIC provides the image data, the image cannot be printed at the desired position or density required.
Thus, the instant inventors needed to invent an architecture which would permit the asynchronous use by a printhead of data produced synchronously by the prior art ASIC image generator.